This invention relates to a fluid displacement apparatus, and more particularly, to an improvement in a rotation preventing device for an orbiting member of a fluid displacement apparatus.
There are several types of fluid displacement apparatus which utilize an orbiting member, such as a piston or a scroll driven by a shaft coupled to an end surface of the orbiting member. One such apparatus, disclosed in U.S. Pat. No. 1,906,142 issued to John Ekelof, is a rotary machine provided with an annular, eccentrically movable piston adapted to act within an annular cylinder provided with a radial transverse wall. One end of the cylinder wall is fixedly mounted and the other cylinder wall consists of a cover or disc connected to the annular piston, which is driven by a crank shaft. Other prior art apparatus, which consist of a scroll type fluid displacement apparatus, are shown in U.S. Pat. Nos. 801,182 and 3,500,119. Though the present invention applies to either type of fluid displacement apparatus, i.e., using either an annular piston or a scroll type piston, for purposes of illustration and not limitation the description of the invention will be set in the context of a scroll type compressor.
The above mentioned U.S. Pat. No. 801,183 (Creux) discloses a device including two scrolls each having a circular end plate and a spiroidal or involute spiral element. These scrolls are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets increases or decreases dependent on the direction of the orbital motion. The scroll type fluid displacement is thus applicable to compress, expand or pump fluids.
Generally, in the conventional scroll type fluid displacement apparatus, one scroll is fixed to a housing of the apparatus and the other scroll, which is the orbiting scroll, is eccentrically supported on a crank pin of a rotating shaft to cause orbital motion. The scroll type apparatus also includes a rotation preventing device which prevents rotation of the orbiting scroll to thereby maintain the scroll in a predetermined angular relationship during operation of the apparatus.
Because the orbiting scroll in conventional scroll type apparatus is supported on the crank pin in a cantilever manner, the orbiting scroll may be subject to an axial slant or tilt. An axial slant occurs also because the movement of the orbiting scroll is not rotary motion, but rather orbiting motion caused by the eccentric movement of a crank pin driven by the rotation of the drive shaft. Several problems result from the occurrence of this axial slant, including improper sealing of line contacts, vibration of the apparatus during operation, and noise caused by physical striking of the spiral elements. One simple and direct solution to these problems is the use of a thrust bearing device for carrying the axial loads. Thus, conventionally, a scroll type fluid displacement apparatus is usually provided with a thrust bearing device within the housing.
One recent attempt to improve the rotation preventing and thrust bearing devices in scroll type fluid displacement apparatus is described in U.S. Pat. Nos. 4,160,629 and 4,259,043 both of which are issued to Hidden et al. The rotation preventing and thrust bearing functions in these U.S. patents are integral with one another. The rotation preventing/thrust bearing device according to these patents comprises one set of indentations formed on the end surface of the circular end plate of the orbiting scroll and a second set of indentations formed on the end surface of the fixed plate attached to the housing. A plurality of balls or spheres are placed between the indentations of both surfaces. All the indentations have the same cross-sectional configuration and the centers of all the indentations formed on both end surfaces are located on circles having the same radius.
In this construction of the rotation preventing/thrust bearing device, during the operation of the apparatus, the balls are held within the indentations by the edge of opposing pairs of indentations. Thus, the rotation of the orbiting scroll is prevented by the balls, while the angular relationship between both scrolls is maintained. Furthermore, during operation each ball is in contact with both end surfaces and rolls along both indentations, so that the axial load from the orbiting scroll, caused by the reaction force of the compressed gas, is carried by one surface through the balls. Since the contact between ball and end surface is formed as a point contact, the pressure acting against the end surface is increased and separation of the end surfaces is likely to occur.